Non-aqueous electrolyte secondary batteries, which have been utilized as main power sources for mobile communication devices and portable electronic devices in recent years, are characterized by high electromotive force and high energy density. As the positive electrode for non-aqueous electrolyte secondary batteries, lithium-containing composite oxide having a layered structure is mainly used. Particularly, lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) have a potential of 4 V or higher relative to that of metallic lithium.
Attempts to use a manganese oxide having a spinel structure as a positive electrode active material for non-aqueous electrolyte secondary batteries have also been actively made. For example, the use of LiMn2O4, Li4Mn5O12 and Li2Mn4O9 has been proposed.
Attention has been given to LiMn2O4 because it is inexpensive and because reduction in production cost for non-aqueous electrolyte secondary batteries is achieved. When a battery containing LiMn2O4 is repeatedly charged and discharged at around 3 V, however, the discharge capacity decreases significantly. This is presumably due to a change of crystal structure caused by Jahn-Teller distortion.
The crystal structures of Li4Mn5O12 and Li2Mn4O9, on the other hand, are relatively unlikely to undergo Jahn-Teller distortion. The crystal structure of Li2Mn4O9, in particular, has vacancy in the cation sites, and therefore Li2Mn4O9 has high capacity density. However, the discharge capacity decreases significantly when these oxides are repeatedly charged and discharged at around 3 V.
U.S. Pat. No. 5,316,877 proposes the use of LiDx/bMn2−xO4+δ, where D is a metal element having an oxidation number of one or greater, 0<x≦0.33, 0≦δ<0.5, and b represents the oxidation number of element D. This document proposes that the element D be Li, Mg or Co. According to this proposal, it is possible to prevent the decrease of discharge capacity during repetitive charge/discharge cycles at around 4 V, but the discharge capacity during repetitive charge/discharge cycles at around 3 V is not sufficient.
In repetitive charge/discharge cycles at around 4 V, the decrease of discharge capacity is caused by the dissolution of Mn2+. Mn2+ is produced through a disproportionation reaction (Mn3+→Mn4++Mn2+). A part of Mn in the manganese oxide having a spinel structure is replaced with Li, Mg or Co, whereby the valence of Mn increases. Accordingly, Mn3+ decreases, which prevents the discharge capacity from decreasing.
In repetitive charge/discharge cycles at around 3 V, on the other hand, the decrease of discharge capacity is caused by Jahn-Teller distortion. The Jahn-Teller distortion is considered to occur when the valence of Mn decreases to 3.5 or less. A part of Mn in the manganese oxide having a spinel structure is replaced with Li, Mg or Co, whereby the valence of Mn increases, and thus the Jahn-Teller distortion is prevented. Even after the element replacement, however, the distortion still occurs to some extent. In other words, it is difficult to prevent the decrease of discharge capacity caused by Jahn-Teller distortion.